The present invention addresses an apparatus and a method which enable smoothly covering a surface of a substrate with a liquid or a substance dissolved in a liquid, and in particular a concept which enables uniformly applying a chemical coating to a substrate. In particular, one-sided coating, etching, cleaning, drying of flat objects such as, for example, plate-shaped panes of glass or flexible materials is made possible.
Applications wherein carrier substrates may be covered with a thin layer of an additional material are manifold. In this context, the additionally applied layer may be active, for example, i.e. exhibit, e.g., an optical or electrical function. Examples of this are application of a photosensitive layer in the production of solar cells or application of a thin phosphorus layer onto a CCD so as to provide same with a conversion layer, so that the CCD in combination with the conversion layer will also be sensitive to X-radiation. In the event of smooth coating with thin layers performing no active functions, the coating frequently serves as a mechanical protection, as is the case, for example, with audio CDs. Here, after production of the CD, a protective layer of a transparent synthetic resin is applied to the optically readable side of the CCD so as to protect same against damage. In this context, the layer thickness of the protective layer may be applied as smoothly as possible over the entire blank CD so as not to influence, in dependence on the position, the optical properties with regard to, e.g., absorption and reflection behavior of a CCD.
When applying optically or electrically active layers, too, the smoothness of the application, or adherence to a specific desired layer thickness is a major objective, since the layer thickness or its homogeneity has an immediate influence on, e.g., the optical or electrical parameters of a component produced by means of coating.
In lithographic methods which include processing a semiconductor surface by means of etching, it is essential that the semiconductor surface may be covered in a controlled manner with an etchant at a uniform thickness and, so that progression of the etching is effected at the same speed over the entire surface area of the semiconductor to be processed.
Conventionally, one has known various methods of achieving smooth coating of a surface. With CDs, for example, the surface to be coated is made to rotate fast, a material used for the coating then being applied in the vicinity of the axis of rotation, so that the material is automatically distributed, by the centrifugal forces, on the surface of the disk, to which it adheres in a uniform layer thickness on account of adhesive forces. Further methods known are, for example, electroplating, i.e. electrochemical deposition of ions which are dissolved in a liquid onto a surface as well as spraying a surface or dipping surfaces to be coated into a bath of the material used for coating.
With chemical methods based on that at least two reagents, which may form, by chemical reaction, the material used for coating, are applied to the surface of a substrate so that, because of the chemical reaction, the material will deposit on the surface, a number of further basic conditions are to be observed. For one thing, the chemical reaction forming the coating material does not take place on the surface only, but also within the volume of liquid formed of the reagents mixed. Depending on the reaction rate, it is therefore at least inconvenient or even impossible to keep a large stock of premixed reagents so as to perform, for example, a dipping process since, within the large volume of liquid kept on stock, the reagents will consume themselves, as it were, by reacting. In this manner, valuable reagents will be wasted, on the one hand, while, on the other hand, future coating processes using the consumed mixture of reagents will no longer be possible. The time limit for processing additionally suggests economic use of the mixture of reagents during application to the surface to be coated since it will be difficult to reuse the mixture of reagents for further processing once it flows off or is removed from the surface. Methods wherein a mixture of reagents is distributed on the surface by means of rotation, for example, are therefore disadvantageous since most of the mixture of reagents is removed from or centrifuged off the surface.
A method of achieving accelerated reaction of the reagents not before they reach the surface of the substrate to be coated is described by the international patent publication WO 03/021648 A1, which describes a chemical surface-coating process for forming an ultra-thin semiconducting film of group IIB-VIA components on a substrate. In this context, a premixed liquid composition containing group IIB and group VIA components is disposed on a heated substrate, so that, on account of the heat of the substrate, a heterogeneous reaction between the different group elements of the liquid coating is enabled. The reaction on the surface of the substrate is accelerated by supplying thermal energy.